The term "supplement" has several meanings, depending on its context.
In a general sense, it refers to something that is added to complete or improve something else. For example, a supplement could be taken as an addition to one's diet in order to provide essential nutrients missing from their regular food intake. In this case, the term "supplement" means an additional substance or aid provided to enhance or complement something else.
In another context, the word "supplement" can refer to a document that provides additional information or clarification about something. For instance, a dictionary may contain supplementary material in the form of appendices or indexes that provide more detailed explanations and references for certain words or topics. In this sense, the term means an addition or enhancement intended to improve or expand upon existing materials.
In summary, "supplement" can mean something added to complete or improve a situation or thing, as well as additional information provided to clarify or enhance existing material.